The present invention relates to a double metal cyanide complex catalyst for ring opening polymerization of alkylene oxide, and its production process.
Heretofore, a polyether polyol useful as a material for e.g. a polyurethane elastomer, an adhesive, a coating, a polyurethane foam, etc., has been produced by polymerization of an alkylene oxide such as ethylene oxide or propylene oxide by using an initiator having active hydrogen. As typical polymerization catalysts of the alkylene oxide, an alkali metal (anionic polymerization), a BF3 etherate (cationic polymerization) and a double metal cyanide complex catalyst (hereinafter referred to as DMC catalyst) (coordination polymerization) have been well known. Among these catalysts, with the DMC catalyst as compared with the former two catalysts, the polymerization activity is high, a cyclic ether compound such as a dioxane derivative will not be produced as a by-product, and a reaction which produces a monool having an unsaturated bond as a by-product is less likely to take place, whereby production of a polyol having a high molecular weight is possible.
The DMC catalyst has been found quite some time ago (U.S. Pat. No. 3,278,457). In recent years, it has been proposed that among organic ligands for the DMC catalyst, tert-butyl alcohol is particularly excellent (JP-A-4-145123, JP-A-8-311171). Further, various modifications regarding the DMC catalyst have successively been proposed. When tert-butyl alcohol is used, the life of the catalyst tends to be remarkably long, and the catalyst remaining in the product polyol can be decreased to a level of at most several 10 ppm even without purification.
The DMC catalyst is a catalyst which is insoluble in water, which contains an organic ligand, water and a metal salt. A typical example of the DMC catalyst excellent in polymerization property is zinc hexacyanocobaltate (Zn3[Co(CN)6]2) containing an organic ligand, water and zinc chloride. A typical production method of such a DMC catalyst will be shown taking zinc hexacyanocobaltate as an example. As shown in the formula (1), an aqueous solution of excess zinc chloride and an aqueous solution of an alkali metal hexacyanocobaltate are mixed in the presence of an organic ligand, so that a solid of a double metal cyanide compound, in which the organic ligand, water and zinc chloride are combined by coordinate bonds, is precipitated, and the solid is washed with a mixed solution of water and the organic ligand and dried to obtain a powdery solid DMC catalyst:
ZnCl2+K3Co(CN)6(H2O/Ligand)xe2x86x92Zn3[Co(CN)6]2X(ZnCl2)Y(H2O)Z(Ligand)+KClxe2x80x83xe2x80x83(1)
Otherwise, an aqueous solution of excess zinc chloride containing no organic ligand and an aqueous solution of an alkali metal hexacyanocobaltate are mixed so that a solid of a double metal cyanide compound is precipitated, and before the solid is separated by e.g. filtration or centrifugal separation, an organic ligand or an organic ligand-containing aqueous solution is added to the mixed solution. If no organic ligand is used at all, the activity as a polymerization catalyst is very low, or no activity is shown at all (U.S. Pat. No. 5,470,813).
The type of the organic ligand, and the ratios of the organic ligand, water and zinc chloride to zinc hexacyanocobaltate are important factors which have an impact on the catalytic property. In the formula (1), as evident from the chemical structural formula of the DMC catalyst as a product, zinc chloride is used in an amount far greater than the stoichiometric amount based on the alkali metal hexacyanocobaltate (Zn/Co atomic ratio: 1.5), and a water-soluble alkali metal salt produced by ligand exchange as a by-product is dissolved in the mixed solution. It has been proposed to produce an active DMC catalyst by mixing zinc chloride in a stoichiometric amount or less based on the alkali metal hexacyanocobaltate, only under a special condition such that the organic ligand and a compound of Group 2A of the Periodic Table coexist at the time of solid precipitation (U.S. Pat. No. 5,952,261).
The solid insoluble in water, which is precipitated by mixing an aqueous solution of zinc chloride with an aqueous solution of an alkali metal hexacyanocobaltate is separated from the solution by e.g. filtration or centrifugal separation. An alkali metal salt remains in a part of the solid, which has an effect as a poison and may deteriorate polymerization activity (U.S. Pat. No.s 3,278,457, 4,472,560). This unnecessary alkali metal salt may be removed by washing the solid with an organic ligand-containing aqueous solution. The alkali metal salt decreases the catalytic activity, and accordingly it is preferably removed as far as possible by washing the precipitated solid with water, however, if the precipitated solid is washed with water, even zinc chloride necessary for obtaining the catalytic activity may also be removed (U.S. Pat. No. 5,158,922). Accordingly, in many cases, washing is carried out with organic ligand-containing aqueous solutions having different compositions several times so that a certain amount of zinc chloride remains. Accordingly, in industrial production process of the DMC catalyst, it has been required to carry out a separation treatment of zinc chloride and cyanocobaltate in the organic ligand-containing waste at the time of recovery, recycle and disposal of the mixed solution of zinc chloride and the alkali metal hexacyanocobaltate and the solution used for washing the solid, thus increasing the cost.
U.S. Pat. No. 4,472,560 discloses a method of mixing a solid precipitated by mixing an aqueous solution of zinc chloride in an amount about double the stoichiometric amount and an aqueous solution of an alkali metal hexacyanocobaltate, and zinc chloride in a molar ratio of 1:1 in a dimethoxyethane aqueous solution, and evaporating the volatile component to prepare a slurry catalyst (Example 16). The polymerization activity is low, and preferred concentration of dimethoxyethane, the amount of zinc chloride, the temperature, etc. in this catalyst preparation system are not disclosed.
On the other hand, in a case where zinc chloride and an alkali metal hexacyanocobaltate are reacted and the obtained precipitated solid is separated, the double metal cyanide compound slurry solution containing excess zinc chloride, an organic ligand and water is in the form of paste, and separation by filtration of the precipitated solid from the solution tends to be difficult (U.S. Pat. Nos. 4,843,054, 5,158,922, 5,952,261). Further, the slurry solution is in the form of paste even when the separated solid is washed with an aqueous solution containing the organic ligand, and the separation by filtration of the solid from the solution tends to be difficult.
Further, WO01/03831 proposes a method for producing a DMC catalyst in such a manner that zinc chloride and an alkali metal hexacyanocobaltate in a stoichiometric amount are reacted to precipitate a solid, and after the precipitated solid is dried, a small amount of an aqueous solution of the organic ligand and zinc chloride in an atomic ratio of at most 1.5 based on cobalt is impregnated in the solid and held at room temperature or under a slightly heated condition, followed by drying. However, no adequate catalytic activity can be obtained, the molecular weight distribution of the produced polyol tends to be broad, and the viscosity of the polyol tends to be high.
A conventionally proposed DMC catalyst has a high activity, and when a polyether polyol is produced by using such a catalyst, production of a polyether monool having an unsaturated bond as a by-product can be suppressed, however, as mentioned above, production of the catalyst tends to be complicated and take long, and accordingly the catalyst cost tends to increase. Accordingly, a process for producing a DMC catalyst which can produce a polyether polyol having a low degree of unsaturation with a high activity easily in a short period of time, has been required.
The present inventors have conducted extensive studies to achieve the above object and as a result, they have found to use a substantially crystalline double metal cyanide compound as a starting material. When the compound is substantially crystalline, filtration at the time of its preparation tends to be easy, and thus the compound is suitable as a starting material. The substantially crystalline double metal cyanide compound is subjected to a contact treatment by heating with an aqueous solution containing an organic ligand and a metal salt, and the resulting solid is separated from the obtained slurry. The present inventors have found a DMC catalyst for ring opening polymerization of alkylene oxide capable of producing a polyether polyol having a low degree of unsaturation with an excessively high activity, produced by carrying out the contact treatment by heating at a specific temperature range with a specific organic ligand concentration, and its production process. The present invention has been accomplished on the basis of this discovery.
Namely, the present invention is to provide a process for producing a DMC catalyst for ring opening polymerization of alkylene oxide, which comprises subjecting a substantially crystalline double metal cyanide compound to a contact treatment by heating with an aqueous solution containing an organic ligand and a metal salt, and then separating the resulting solid from the obtained slurry.
The present invention further provides a DMC catalyst for ring opening polymerization of alkylene oxide, obtained by the above process.